How to rewrite code for IE?

Question

JavaScript

function charIndexSort(a,m){
  a=a.toLowerCase();
  return m.slice(0).sort(function(name1,name2){
    let i1=name1.toLowerCase().indexOf(a);
    let i2=name2.toLowerCase().indexOf(a);
    if(i1<0)return 1;
    if(i2<0)return -1;
    return Math.sign(i1-i2);
  });
}

IE 11 throws an error
Object does not support property or method "sign"
How to rewrite the code in this case?

Answer 1

function charIndexSort(a,m){
  a=a.toLowerCase();
  return m.slice(0).sort(function(name1,name2){
    let i1=name1.toLowerCase().indexOf(a);
    let i2=name2.toLowerCase().indexOf(a);
    if(i1<0)return 1;
    if(i2<0)return -1;
    return 0;
  });
}